Heat exchanger or solar collector

ABSTRACT

A heat exchanger unit comprising a particulate heat exchanging mass or pack consisting of mechanically immobilized particles having no more than 750 microns mean diameter and having a thermal diffusivity constant of at least 0.5 cm 2  /sec at 20° C., and compressively retained in an enclosure in heat transfer relationship to each other and to a fluid directed therethrough. Preferred materials for the particles are crystalline carbon, copper and aluminum. The pack may be in cylindrical form or planar form and may be contained within metal conduits or, for solar radiation, within a transparent or translucent enclosure. Interconnected units may be disposed in an array or bank to provide the desired quantitive degree of thermal transfer.

This application is a continuation-in-part of my prior copendingapplication Ser. No. 06/225,254 filed Jan. 15, 1981 and now abandoned.

BACKGROUND OF THE INVENTION

The rate of heat transfer in a heat exchanger is a function of severalfactors, regardless of whether the exchanger is for cooling purposes,for heating purposes, for transfer between fluids, or for collection andtransfer of radiant energy. The rate of heat transfer is determinativeof the efficiency, as well as the level or quality of performance of theheat exchanger for its intended purpose.

The conversion of radiant energy to heat involves the absorption of asmany wave lengths of the radiation as possible by a black (or grey) bodyand transfer of heat from the black body to a working fluid such aswater or air.

The rate of heat conduction, dQ/dt, from the transfer body of a heatexchanger to the working fluid is governed by the equation: ##EQU1##where Q is energy units, t is time, K is the limiting heat conductivityconstant of either the fluid or the exchanger body, whichever is less,A/L is a fraction in which A is the area of contact between the transferbody and fluid, L is the length or distance between the hot surface ofthe heat absorbing and exchanging material and the coolest or medianportion of the adjacent body of working fluid; Ta is the temperature ofthe contact surface of the transfer body; and Tf is the temperature ofthe fluid at the aforesaid median portion. The fraction A/L is ofparticular interest in connection with the present application.

Specifically, for a given value of Ta-Tf, K is a constant characteristicof the fluid and therefore the rate of conduction, (dQ/dt) is directlyproportional to A/L. Consequently, a main objective of heat exchangerdesign is to make A as large as possible and L as small as possible.This has resulted in elaborate tubular flow arrangements, a commonexample being the conventional automobile radiator. In the usual case,when water or air is the working fluid, K is small and the heatexchangers rely not only on conduction, but also on natural or forcedconvection of the fluids to induce heat transfer.

A common feature or element of such heat exchangers is illustratedschematically in FIG. 1 of the accompanying drawings which are part ofthe present application, and in which the fluid is made to circulatewithin a metallic tube or conduit 1 which is attached to the surface ofa metallic member or fin 2. Heat exchange, in this instance, depends onconduction of heat to or from the fluid to the surface of the transfermember or fin 2. It will be understood that, for industrial use, manyhundreds or thousands of such tubes would be utilized. When such anexchanger is used as a solar collector, the surfaces facing the sun aremade black by painting, oxidizing, etc. Thermal insulation for such anexchanger from its surroundings is provided by one or more transparentcovers 3, 4 and a backing 5.

Another prior art form of tubular heat exchanger is schematicallyillustrated in FIG. 2 of the drawings. In this form the heat transfertakes place between fluids circulating in the inner conduit 6 and thecoaxial outer conduit 7. When this form is used as a solar collector,the working fluid circulates within a blackened inner conduit 6, made ofmetal, which is placed within a transparent tube 7; the space betweenthe conduit 6 and tube 7 being evacuated to reduce heat loss byconduction and convection.

Typically, the radii of tubes used in solar collectors are of the orderof 1 cm. Therefore, A amounts to approximately 6 cm² for each linearcentimeter of tube. Since L is approximately 1 cm, A/L is 6 cm per unitlength of tube.

The art has also addressed itself to non-metallic heat exchangers orsolar collectors. Representative of the current state of the art is therecently issued Rice et al, U.S. Pat. No. 4,310,747 and the prior artHarvey U.S. Pat. Nos. 4,082,082 and 4,129,117.

Rice et al disclose a heat transfer device in which the heat source iseither an electrical resistance element or, alternatively, solar energy,or possibly both in combination. The same heat exchanging material isutilized for both sources of energy and consists of a baked, skeletal,porous, vitreous carbon structure containing multi-directional,inter-connected carbon strands having electrical continuity. Thestarting material for this skeletal network is a flexible polyurethaneresin reticulate structure which is transformed into non-crystallineamorphous carbon. Rice et al describe this porous body as having adensity of about 0.05 g/cc. Inasmuch as amorphous carbon has a densityof about 2.0 g/cc, it is evident that the composite body is highlyporous and would have a ratio of carbon to space (or flow passages) onthe order of 1:40 providing a relatively small carbon mass and contactarea for heat transfer contact with the working fluid, although,conversely, providing a relatively large area of flow passageways forthe fluid.

It is also to be noted that vitreous amorphous carbon, such as utilizedby Rice et al, not only has significant electrical resistivity toachieve the patentee's objective of providing an electrical resistanceheating element, but also has a relatively low value of thermaldiffusivity or conductivity on the order of 1.0×10⁻³ cm² /sec, which issignificantly lower than the thermal diffusivity of the metals, such ascopper, used in tubular heat exchangers, which is on the order of 1.0cm² /sec.

The two Harvey patents, disclose a solar collector utilizing aparticulate or fibrous blackened exchanging material which ischaracterized by the patentee as having "high solar energy absorptionand low thermal diffusivity (e.g. preferably below 2.5×10⁻³ cm² /sec)".A variety of materials are listed, with carbon filled high densitypolyethylene being a preferred example. The particle size is describedas a mean diameter in the range of 1 to 10 mm, preferably 3 to 4 mm. Theparticles are deliberately loosely packed to permit them to move orcirculate freely in response to flow of the working fluid, presumably topermit sequential exposure of the particles to the uni-directional solarradiation, as they are not packed to be thermally conductive with eachother.

Neither Harvey nor Rice et al disclose any performance data for theirsolar collectors, but both mandate very low values of thermalconductivity for the absorber material, as this characteristic isnecessary to achieve the result the patentees seek. In Harvey, lowthermal diffusivity of the particles is desirable to localize heatabsorption at the surface of the moving particles and increase theexposed particle skin temperature by inhibiting heat transfer to theinterior of the particle. In Rice, very low thermal diffusivity istaught to achieve a carbon strand having high electrical resistivity forresistance heating.

The prior art tubular metal heat exchangers, particularly those usedindustrially or in chemical processing, utilize solid metal heatexchange bodies which have relatively high thermal diffusivity, but areunduly restricted in area of contact with the working fluid by reason ofmechanical design limitations.

The prior art non-metallic collector of the Harvey patents attempts toenhance the area of heat transfer contact by the use ofcarbon-containing particles for the heat exchange medium, but deems itnecessary and desirable to sacrifice good thermal diffusivity to do so.The same is true of the more recent Rice disclosure. Both of thesedisclosures could have limited usefulness in intermittent, low-demandsituations where a low rate of heat transfer is acceptable and adequate,as for supplementary hot water heating for home use.

The present invention is directed to overcoming the limitations of priorart heat exchangers, both the metallic tubular and non-metallic, bysignificant and radical improvement of the rate of heat transfer,particularly in the continuous, high-demand industrial and nuclearapplications, regardless of whether the heat source be solar radiant orfluid conductive in character.

SUMMARY OF THE INVENTION

The invention has as its primary object the provision of a heatexchanger or solar energy collector which radically enhances the rate ofheat transfer, and achieves improved efficiency.

Another object of the invention is to provide a heat exchanger or solarenergy collector of the character described which has high constancy orconsistency, i.e., maintenance of uniformly high efficiency under alloperating conditions.

Another object of the invention is to provide a heat exchanger or solarenergy collector of the character described which, per unit of mass orsize, has a significantly improved response to energy input, i.e., low"thermal inertia".

Another object of the invention is to provide a heat exchanger or solarenergy collector of the character described which is lower in cost thanexchangers of the prior art.

A further object of the invention is to provide a heat exchanger orsolar energy collector of the character described, which is highlydurable, can be operated virtually free of maintenance costs andutilizes durable materials, such as glass and elemental carbon in itsconstruction.

A further object of the invention is to provide a heat exchanger orsolar energy collector of the character described which is of simpleconstruction and which embodies system simplification, i.e., eliminationof other components, such as additional heat exchangers and regulatingequipment in an overall heating system.

A still further object of the invention is to provide a heat exchangeror solar energy collector of the character described in which the use ofadditives such as anti-freeze, corrosion inhibitors, etc., iseliminated.

The foregoing objectives are attained by a novel thermodynamic conceptwhich achieves unexpectedly great increases in the rate of heat transferby means of a structure which combines enhancement of both the contacttransfer area and the thermal conductivity of the heat exchanger unit,with improved initimacy of contact in the heat transfer function.

Other objects, advantages and novel features of the invention willbecome apparent from the following description of the invention whenconsidered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood and the numerous objectsand advantages thereof will become apparent to those skilled in the artby reference to the accompanying drawings, in which:

FIG. 1 is a schematic or diagrammatic illustration of a tubular metallicheat exchanger representative of the prior art;

FIG. 2 is a schematic or diagrammatic illustration of another form oftubular metallic heat exchanger representative of the prior art;

FIG. 3 is a schematic or diagrammatic illustration of a heat exchangeror solar collector embodying one form of the invention;

FIG. 4 is a schematic or diagrammatic illustration of a heat exchangeror solar collector embodying a modified form of the invention of FIG. 3;

FIG. 4A is a schematic or diagrammatic illustration of a heat exchangeror solar collector embodying the invention in a modified form of theprior art tubular exchanger of FIGS. 1 or 2.

FIG. 5 is a graph showing the performance of the collector of FIG. 4 ofthe present invention as a function of the rate of water flow in ml/minat solar incidence normal to the collector surface and amounting to77-80 calories per square centimeter per hour, and illustratesmaintenance of constant high efficiency of heat transfer over a broadrange of flow rates;

FIG. 6 is a graph showing the behavior of the collector of FIG. 4 of thepresent invention under typical working conditions over an entire day atconstant collector orientation and constant water flow, and indicatesthe uniformity of heat transfer efficiency at varying heat input values;

FIG. 7 is a graph showing the performance of the collector of FIG. 4 ofthis invention as compared to the performance indicated by curves 1 and2 in FIG. 7, under conditions of varying fluid flow rate and varyingradiation energy input.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring more particularly to FIG. 3 of the drawings, there is shown athin radiation-absorbing layer 8 composed of particles of crystallinecarbon having relatively high thermal diffusivity, preferably graphite,compressed or packed between a radiation-transmitting cover 9 such astempered glass, and an insulation backing 10 of glass or other rigidinsulating material. The surface 9 is adapted to be exposed to solarradiation.

In assembly, the layer 8 is subjected to pressure sufficient toimmobilize the particles at the operation conditions to which it will besubjected, as well as to maintain the particles in thermally-conductivecontact with each other. For example, if the unit is to be used forwater at ordinary house pressure, a particle pack pressure of about 100lbs/sq. in. would be used; or if higher operating pressures or forcesare anticipated, the pack pressure would be increased to a valuesufficient to prevent movement of the particles relative to each otherat such higher working fluid forces.

The mean diameter of the particles is in the range of 750 microns downto about 50 microns, with 500 microns mean diameter being a preferredmaximum. The objective is to use particles which are as small aspracticable commensurate with the maintenance of ample fluid passagewayinterstices and with the maintenance of discrete particles at the packpressure value to which the particles are subjected. The working fluid,such as water, flows through the spaces between the non-contactingportions of the particles of the layer 8.

The particles in addition to functioning as heat absorbers, serve asexcellent thermal conductors. This thermal conductivity extendsthroughout the particle mass by reason of the pressure contactmaintained between all particles. The thermal conductivity ordiffusivity of graphite in the basal crystallographic plane is about 1.0cm² /sec, which is of the same magnitude as that of the heat exchangerconduit metals, such as copper or aluminum, which are conventionallyconsidered most desirable from a practical and economic standpoint. Theheat diffusivity constant is preferably at least 0.8 cm² /sec at 20° C.,but should not be less than 0.5 cm² /sec at 20° C. to achieve theobjectives of the invention.

The surface area of the particles is typically 1 to 40 m² per gram,depending on the particle size selected within the given range. For anoverall (projected) surface area of collector of 1 cm² and a particlelayer thickness of 0.1 to 0.4 cm, typically containing on the order of0.1 grams of graphite particles, the surface area of particles in directcontact with the fluid, and therefore the value A in the equation, isthen on the order of 10³ to 10⁴ cm², while L, which is herein theaverage locus of the adjacent fluid between the small particles, is ofthe order of a few microns or 10⁻³ to 10⁻⁴ cm. Regardless of theparticular particle size selected within the given size range, themaximum value of L is one-half of the mean diameter of the particle.

Accordingly, the value of the critical factor A/L in the foregoingequation (1) for the described embodiment is on the order of 10⁶ to 10⁸cm, and would be no less than 10⁵ cm at the high end of the range ofparticle size. This unexpectedly high value is a resultant of theseveral factors previously mentioned: (a) the particle size of thegraphite transfer material is small, so as to establish an extremelyhigh ratio of exposed surface area to mass; (b) the particles areimmobilized and packed to maintain contact with each other throughoutthe mass, despite the pressures or forces of fluid flow which wouldotherwise tend to disrupt this inter-particle continuity of contact; (c)the fluid-filled spaces or interstices between contiguous particles isof very small magnitude, so that the distance from the heat transfersurface of any particles to the median point or locus of the immediatelyadjacent body of fluid in contact therewith, which defines L in theequation (1), is correspondingly of very small value; (d) the thermaldiffusivity of the graphite pack is relatively high, thus transferringand conducting the absorbed input heat to all particles throughout theparticle pack, even though only some of the particles are directlyexposed to the radiation energy; and, consequently (e) all of thesurface area of all of the particles presents an effective area of heattransfer contact with the fluid passing through the small passageways ofthe particle pack, thus significantly increasing the value of A in theequation (1) beyond that which exist if only the surfaces of thoseparticles directly exposed to and heated by the radiation energy wereoperatively effective for heat transfer to the adjacent fluid.

Inasmuch as the value of K in the equation (1) is the heat conductivityconstant of water in all the comparative analysis herein made, and thevalue of (Ta-Tf) is only variable within relatively narrow limits, therate of heat transfer, represented by (dQ/dt), is influenced almostentirely by the quantity A/L. The corresponding value (dQ/dt) is of theorder of 10⁶ to 10⁸ ·K cal/cm² ·sec based upon the foregoing values,when Ta-Tf is 1° C. and where K is the established heat diffusivityconstant of water at the attained temperature of the transfer fluid.This compares to a heat transfer rate of 6·K cal/cm² ·sec for the coppertube heat exchanger of FIG. 1 or FIG. 2, using tubes of 2 cm internaldiamater for purposes of calculation, so that L would have a value of 1cm. The value of K would be the same in both instances. The heattransfer rate for a prior art copper tube cluster of extremely smallindividual internal diameter, e.g. 5 mm, might be as great as 1000·Kcal/cm² ·sec.

It also compares to a calculated heat transfer rate of 0.1 to 10.0·Kcal/cm² ·sec for the non-metallic solar collector of Harvey or Rice etal, taking into account the limited area of hot exchanger surfaces forheat transfer in both of these disclosures.

It can be conservatively stated that the inventive embodimenthereinabove described has a heat transfer rate at least 1000 timesgreater than that of the most efficient prior art tubular metallic heatexchangers and at least 100,000 times greater than that of the Harvey orRice disclosures. Thus, the inventive embodiment has such a high rate ofheat transfer that it appears to transfer heat instantaneously, whencompared to the other known forms of heat exchangers above-described.

The basic embodiment of FIG. 3 lends itself readily to manyconfigurations and applications, as will be presently explained, withspecial emphasis to those which can be assembled readily from commonlyavailable durable materials.

Referring more particularly to FIG. 4, a flat plate solar collector isshown, which although it is not optimized with respect to construction,and represents a minimum of performance, proved to fulfill all of theobjectives of the invention, which have hereinabove been stated.

In FIG. 4, reference numeral 8 designates a thin layer of graphiteparticles tightly packed between a cover 9 and a backing 10, the coverand backing being made of tempered glass panels, approximately 20 incheslong, 7 inches wide and 3/16 inch thick.

In assembling the aforesaid structure, a thin bead of black siliconesealant 15 was applied to the marginal edges of three sides of the panel10. At each corner, a small object of desired thickness, such as a pieceof a copper coin or a copper sphere, as for example, a BB gun pellet,was embedded in the silicone bead in order to establish the spacingbetween the panels.

The cover panel 9 was then attached by pressing it down upon thesilicone bead and the assembly was left to cure overnight.

The graphite particles 8 were available as recovered scrap from graphiteelectrode manufacturers, or as commercial products intended for avariety of purposes. These graphite particles were screened for a sizeof 500 microns mean diameter, washed with a degreasing liquid, such asacetone, and then poured into the space between the panels 9 and 10 andpacked in place by applied pressure. The marginal edges of the fourthside of the panels were then sealed with silicone sealant.

Alternatively, particularly when higher particle pack compression isdesired, the sealant 15 is applied to all four marginal edges of thepanel 10; the cover panel 9 is attached, using it as a platen for thedesired pressure to be imposed on the contained particle pack, while, atthe same time, using any form of suitable fixture or rigid retainingframe around the periphery of the assembly to prevent escape of thesealant; and then permitting the sealant to cure.

Inlet and outlet tubes for the fluid should consist of corrosionresistant materials, preferably glass or synthetic resin. In thisinstance, however, these were provided by inserting through the curedsilicone sealant 15, air needles 14 such as commonly used for inflatingfootballs and which, in turn, were connected to input and outputpolyethylene tubing (not shown).

A backing 11 of insulating material was provided consisting of a sheetof polystyrene foam approximately 1/2 inch thick and spot glued to theback of the back panel 10 by means of a few drops of silicone sealant.

A rectangular frame consisting of hardwood dowel stock 13, 3/8 inch indiameter and coated with silicone 16, was attached to the panel 9 and atransparent cover 12 of insulating material was attached to the siliconecoating 16 of the frame 13, the silicone serving both as a preservativecoating for the frame and as a sealant between the frame and the covers12 and 9.

The distance between the panels 9 and 10 was approximately 0.4 cm.

The performance characteristic of the embodiment shown in FIG. 4 incomparison with the prior art is graphically depicted in FIGS. 5, 6 and7 of the drawings.

The test data in FIGS. 5, 6, and 7 was obtained by using water as thefluid. The water was contained in a reservoir approximately 1 meterhigher than the collector, and the water flow was controlled or adjustedby means of a common stop-cock or valve. The inlet and outlet watertemperatures were measured with mercury thermometers, as were thetemperatures of the panel 10 and the cover 12.

The incident solar energy was measured at the plane of the collector byplacing on cover panel 12 a calibrated solar meter (Dodge Products,Houston, Tex., Model 776). The area of the collector was measured andfound to be approximately 860 cm².

FIG. 5 shows the performance of the collector as a function of the rateof water flow in ml/min at solar incidence normal to the collectorsurface and amounting to 77-80 cal/cm² ·hour. It is seen that theefficiency, calculated as: ##EQU2## rises rapidly to 80% at the minimumdesign flow rate of approximately 15 ml/min and remains substantiallyconstant thereafter in a broad range of flow rates within theexperimental error indicated by the error bars. This is in markedcontrast with the behavior of typical collectors of the prior art inwhich the efficiency is maximum at zero flow and decreases continuouslythereafter (see FIG. 7). The initially low percentage of efficiencybelow the design limit of 15 ml/min is due to the fact that at these lowflow rates the water is not distributed over the entire surface of thecollector but prefers to follow clearly visible random channels. Inapplications where such relatively slow flow rates are sufficient,channeling can be prevented by the use of smaller graphite particlesand/or a thinner layer of graphite.

It is further seen in FIG. 5 that in this model, temperaturesapproaching 90° C. can be obtained, and the design temperature of 60° C.(household hot water temperature) can be obtained under these conditionsat 30 ml/min.

FIG. 6 shows the behavior under typical working conditions over anentire day at constant collector orientation and constant water flow.

The collector was oriented parallel to the house roof facing southwestat an inclination to the horizontal plane of 20-25 degrees. The flowrate was set at 19-20 ml/min and monitored continuously.

The test date in FIG. 6 demonstrates that collector efficiency issubstantially constant also with varying angle of solar incidence, andtherefore with the amount of solar energy incident on the collector, asplotted, except at times of extremely low angles of incidence, namely at0900 and 1800 hours. The irregularities in the temperature and incidentsolar energy curves after 1530 hours are due to intermittent cloudinessoccurring after this time.

It is noted that the efficiency is less than 100%. It was determined, bymeasuring the incident solar energy with and without panels 9 and 12 infront of the solar meter, that this reduction in efficiency is duealmost entirely to reflection-absorption by the panels. This is rathersurprising because normally, appreciable loss is caused byconvection-conduction to the surroundings from the hot collector.Apparently, this is minimized in this collector by the thinconfiguration of the heat-exchanging absorber and this is corroboratedby the fact that the efficiency does not vary with the temperature ofthe collector, as seen in FIGS. 5 and 6.

In scientific and technical literature it is customary to express thecharacteristic performance of the collectors by plotting efficiency onthe ordinate versus the fraction ##EQU3## on the abscissa, whereTf=fluid outlet temperature °C.

Ta=fluid inlet temperature °C.

I=rate of solar energy input in watts/m²

resulting in efficiency expressed as a function of ##EQU4## Such plotsare then used in order to compare various collectors as in FIG. 7.

In order to compare the collector of this invention with those of priorart, authoritative plots of this kind are used and reproduced in FIG. 7.These were obtained for well known commercial collectors by the NationalBureau of Standards and published in Solar Energy, volume 18, page 421(1976). Further work on comparing collectors by means of these plots isdescribed in the same issue of that journal, page 451 by NASA LewisResearch Center.

In FIG. 7 the best performance is reported in the above publications for(a) a two-cover glass collector with a Mylar honey comb insert, curve 1in FIG. 7, representing an elaboration of FIG. 1 in this disclosure and(b) an evacuated cylindrical collector with a selective coated absorber,curve 2 in FIG. 7, representing an elaboration of FIG. 2 in thisdisclosure.

Numerous publications and advertisements since 1976 (not reproduced inFIG. 7) show little if any improvement over those of curves 1 and 2 inFIG. 7.

In comparison to the performance indicated by curves 1 and 2, theperformance of the collector of FIG. 4, curve 3 in FIG. 7, shows twostriking differences: (1) the efficiency is substantially constant and(2) the efficiency is substantially higher.

Another unit or model similar to the one described above was placed inthe freezer compartment of a household refrigerator after it had beenfilled with water and exposed to the sun. The water froze but there wasno cracking or other damage to the unit. Apparently, the elasticity ofthe silicone sealant and/or the flat glass panel is adequate to allowsufficient expansion to accommodate the increase of volume of water uponfreezing. Therefore, for this collector, it will not be necessary to usean anti-freeze, as is necessary for collectors of the prior art.

Various modifications may be made in the collectors described above, asindicated in the following examples.

For example, the tempered glass for these units was high in iron content(green glass). Using iron-free tempered glass will increase transmissionto approximately 90%.

The backing 11 of the collector can be made of concrete foam, asdisclosed in applicant's U.S. Pat. No. 4,267,021 on solar desalination.This should eliminate the need for using less durable insulation, suchas that made of organic materials.

The life of the silicone sealant was found to be between 5 and 10 years.The front and back panels can be joined by fusing a glass shim on thepanels in order to eliminate the organic sealant.

These units can be joined together to constitute part of the house roof,instead of resting on the shingles.

The cover 12 can be a common surface for all of the units instead ofbeing attached to each individual unit. The distance between the covers9 and 12 can be optimized, as taught in U.S. Pat. No. 4,267,021, towhich reference has been made above.

The heat exchangers or collectors of the present invention renderpossible the elimination of other heat exchangers and equipment usedtherewith.

The water pressure in solar collectors of the prior art is much lowerthan that of normal city water pressure. The heated water is circulatedby a pump through the collector and then down to a heat exchangerimmersed in the hot water tank of the house.

It was found that the embodiments or model of the present invention, asdescribed above, will withstand normal city water pressure (exceeding 40lbs/in² in the locale of the residence of the applicant) if they areclamped at the edges by U-Shaped aluminum alloy conduits. This makes itunnecessary to use the additional heat exchanger. If it is necessary touse glass of lesser thickness than the 3/16 inch thick glass used here,the backing of these units can be made to be the top of a thin metallicbox also filled with graphite particles, through which the high pressurecity water can circulate with efficient heat exchange occurring betweenthe low pressure water in the collector and the high pressure water inthe lower metallic box. Finally, if it becomes necessary to retain theheat exchanger in the hot water tank, this heat exchanger can beidentical with the metallic box described above, but operating in serieswith the collector, i.e., containing the heating water at the reducedpressure.

Instead of glass members 9 and 10 as in FIG. 4, porous firebrick couldbe used, the surfaces of which are made impermeable by a coating ofappropriate cements. The cement coating facing the sun is made black byincorporating in it common cement plus a dye or carbon black. In thiscase, although the fraction A/L of equation (1) is still very large, thevalue of K for the firebrick is low and the "thermal inertia" of thesystem isalso very large. Such an embodiment serves the function othermal storage because once the system is heated it reains hot fordays. Alternatively, or additionally, the waer heated by embodimentssuch as that of FIG. 4, can be onducted to and circulated through a bedof porous firebrck which thus functions as a means of storing heat forextended periods of time, e.g., weeks.

As shown in FIG. 4A heat exchanger configurations of the tubularmetallic type, such as shown in FIG. 1 and 2 for example, also lendthemselves to radical improvement in the rate of heat transfer throughuse of my inventive concept. Instead of using the conventional approachof utilizing the interior of conduit 1 or conduit 6 solely as a fluidpassageway, these passageways are packed with particulate graphite inparticle sizes and at packing pressures in the range previouslydiscussed. This structure provides a thermally conductive mass withinthe conduit whose thermal diffusivity is of substantially the samemagnitude as the copper or aluminum in which it is encased in heattransfer contact. This heat exchanger mass provides a significantlylarger area of heat transfer contact between the particles and the fluidpassing through the mass, as well as a multiplicity of minute flowchannels to direct the fluid into intimate contact with adjacent heattransfer particles.

As a consequence, the value of the quantity A in the equation (1) isgreatly enlarged and the value of the quantity L is greatly diminishedto enlarge the value of A/L. This results in a rate of heat transfer atleast 1000 times greater than could be obtained without the introductionof the particles in the flow conduit.

Similarly, the annular space between conduits 6 and 7 of FIG. 2 ispacked with graphite particles having a thermal diffusivity which is ofcomparable magnitude to that of the encasing metal tube. Such anarrangement further improves the rate of heat transfer through thecounter-flow fluid passing through the annular space, for the reasonsabove mentioned.

The use of small diameter tube sizes in heat exchangers has beenconsidered desirable primarily to enhance the area of heat transfercontact with the fluid. Inasmuch as the packed particles can be reliedupon for that desirable characteristic, it is no longer necessary forexcellent and improved heat transfer to minimize the metal conduitdiameters. These diameters can be enlarged to accommodate the desiredrate of fluid flow through the particle pack, without doing anynoticeable injury to the radical and unexpected improvement obtained inthe rate of heat transfer and in efficiency.

In those applications where corrosion is not a significant problem,metallic particles, such as copper or aluminum may be utilized insteadof and in the same manner as described above for graphite, to achieveimproved results of the same magnitude. There also may be circumstanceswhere the graphite and the metal particles are desirably used incombination. For convenience and simplicity the foregoing disclosure hasbeen directed principally to solar heat exchangers or collectors, but itis to be understood that the invention is not limited to radiant orsolar energy heat exchange, but is adapted for general use in heatexchangers for fluids utlizing conduction and/or convection heat inputor output, as well as radiation. Banks or arrays of multipleinter-connected units may be utilized to achieve the quantitative resultdesired.

It is to be understood that the forms of my invention, herewith shown ordescribed, are to be taken as preferred examples of the same, and thatvarious changes in the shape, size and arrangement of parts may beresorted to, without departing from the spirit of my invention, or thescope of the subjoined claims.

Having thus described my invention, I claim:
 1. In a solar collectorunit, the combination of a heat transfer pack consisting of compresseddiscrete particles of radiation energy-absorbing solid matter having amean diameter of no more than 750 microns and having a thermaldiffusivity constant of at least 0.5 cm² /sec at 20° C., said particlesbeing immobilized relatively to each other by a pack pressure sufficientto maintain said particles in physical and thermally-conductive contactwith each other during operating conditions, said particles providing aplurality of interstitial fluid passageways through said pack, and anenclosure for retaining said particles in solar radiationenergy-absorbing relationship and in heat transfer contact with a fluiddirected through said pack.
 2. A combination as defined in claim 1, inwhich said pack is substantially cylindrical in form.
 3. a combinationas defined in claim 1, in which said pack is substantially planar inform.
 4. A combination as defined in claim 1, in which said solid matteris selected from the group consisting of crystalline carbon and metals.5. A combination as defined in claim 4, in which said enclosure includesat least one face or area particularly adapted for solar radiation wavetransmission to said heat transfer pack.
 6. A combination as defined inclaim 4, in which said enclosure is a tubular conduit.
 7. A combinationas defined in claim 4, in which said enclosure is a first conduitcontaining a first heat transfer pack and at least a second conduitcoaxial with said first conduit and containing a second heat transferpack.
 8. A combination as defined in claim 4, in which said carbon is inthe form of graphite and said metals are aluminum and copper.
 9. Acombination as defined in claim 1, where the quantity A/L in theequation (1) herein has a value of at least 1×10⁵ cm for said heattransfer pack.
 10. A combination as defined in claim 1, wherein saidparticles are maintained at a pack pressure greater than the pressure ofthe fluid flowing through said pack.
 11. A combination as defined inclaim 10, wherein said pack pressure is at least 40 lbs./in².
 12. In asolar collector unit, the combination of a heat transfer pack consistingof compressed discrete particles of radiation energy-absorbing solidmatter having a thermal diffusivity constant of at least 0.5 mc² /set at20° C., said particles being immobilized relatively to each other by apack pressure sufficient to maintain said particles in physical andthermally-conductive contact with each other during operatingconditions, said particles providing a plurality of interstitial fluidpassageways through said pack, the average locus of said fluidpassageways between said particles having a maximum value no greaterthan one-half the particle size mean diameter, and an enclosure forretaining said particles in solar radiation energy-absorbingrelationship and in heat transfer contact with a fluid directed throughsaid pack.
 13. A combination as defined in claim 12, wherein saiddiscrete particles have a mean diameter of no more than 750 microns. 14.The method of improving the rate of heat transfer in a solar collector,comprising the steps of:(a) packing the fluid passageway of said solarcollector with minute discrete particles of radiation energy-absorbingsolid material having a relatively high thermal diffusivity constant,(b) maintaining said particles at a pack pressure sufficient toimmobilize said particles in physical and thermally-conductive contactwith each other during fluid flow, (c) maintaining said particles inheat transfer contact with each other, and (d) providing a multiplicityof new interstitial fluid passageways through said packed particles forheat transfer contact of the fluid with said particles.
 15. The methodas defined in claim 14, wherein said particles have a thermaldiffusivity constant of at least 0.5 cm² /sec at 20° C.
 16. The methodas defined in claim 14, wherein said particles have a mean diameter ofno more than 750 microns.
 17. The method as defined in claim 16, whereinsaid particles have a thermal diffusivity constant of at least 0.5 cm²/sec at 20° C.
 18. The method as defined in claim 16, wherein theaverage locus of said fluid passageways between said particles has amaximum value no greater than one-half the particle size mean diameter.